1. Field of the Art
This invention relates to a method of forming a metal film consisting mainly of copper, on a ceramic body, and more particularly to a method of forming a copper film which is free from blisters or bubbles and which adheres to the ceramic body with high adhesion, providing a complete coverage even where the substrate surface of the ceramic body is complicated with a through(hole or the like.
2. Related Art Statement
There are known a number of methods for forming a copper film on a ceramic body. They include chemical solution deposition (electroless plating) of copper, and its vapor deposition, such as vacuum evaporation, sputtering or ion plating. None of these methods, however, are satisfactory in forming a film of copper which firmly adheres to the ceramic substrate or body.
British Patent No. 2 141 741A proposes a method for forming a copper film of improved adhesion to a ceramic substrate. The proposed method utilizes the so-called "anchor effect" which is attained by etching the substrate surface as by alkali fusion. However, this method is difficult to employ for high-yield large-scale production of ceramic bodies carrying copper films with high adhesion, since the etching condition is difficult to control, and is likely to damage the surface layer of the ceramic substrate.
A method for metallizing a ceramic body with copper, which is known as the direct copper bonding technique, is disclosed in a in Japanese Patent Application which was published in 1985 under Publication No. 60-4154 for opposition purpose. According to this method, a relatively thick copper sheet or foil having a very thin surface layer of copper oxide is bonded to a ceramic body by heating at a temperature of 1065.degree. C. to 1083.degree. C. in a nitrogen atmosphere, or at a temperature of 1065.degree. C. to 1083.degree. C. in a nitrogen atmosphere containing a very small amount of oxygen. This method has, however, a lot of disadvantages. The temperature range for the heat treatment and the allowable range of oxygen concentration in the atmosphere are too narrow to be easily attained. During the heat treatment, the copper foil does not deform sufficiently so as to fit the substrate surface snugly, thus failing to easily adhere to the surface. The method cannot be employed for metallizing the surfaces defining holes formed in the substrate, and its application is limited to ceramic bodies having a flat substrate surface. Further, the method in question requires the copper foil to have a relatively large thickness for easy hardling and heat treatment, and the copper film formed by this method cannot be suitably processed by photolithography to form a high-density wiring pattern.
There is also known a copper plating method which relies on the anchor effect of a porous layer, such as porous glass, which is applied to the article such as a ceramic substrate. The porous glass is obtained, for example, by firing glass-containing paste on the substrate. This method has the disadvantage that the presence of the porous layer prevents copper plating of through-hole surfaces, and lowers the thermal conduction between the metal film and the substrate.